Sunday, November 20, 2011

What is an Identifying Characteristic?

A definition describes a concept.  A concept has many characteristics.  The process of abstraction occurs when we consider the characteristics of a concept individually (or, at least this is one way in which the term "abstraction" is used).  Very often, the characters considered the most when doing definitions are those which describe the essence of the concept.  Consider, for instance, the traditional Aristotelian formula for a definition:

Definition = Genus + Specific Difference

In the historical literature about definitions, emphasis was put on the specific difference as being the essential characters of a concept.  This may serve the philosophers' purposes, but it ignores the problem of identification, and the need to consider identifying characteristics in a definition.

Suppose I want to define the concept of Exit Row in a plane.  Let's try to do so using Aristotle's method.

"A row of seats that provides emergency access to the outside of a plane"

A bit provisional perhaps, but is does begin to convey the essence of an Exit Row.  But how do flight attendants identify an Exit Row to passengers when they give the safety briefing?  It goes something like this:

"For those of you  seated in a row marked by the sign 'No Children In This Row'..."

The essence of an Exit Row is not that it is an area of a plane that is kept free of children.  The essence is that it is a point of emergency egress from a plane.  Only competent adults are thought to be capable of opening an emergency exit, so only they are permitted to sit in an Exit Row.

However, it fairly easy for a passenger  to identify an Exit Row based on finding a sign saying 'No Children in This Row'.  The sign functions as an identifying characteristic.

An identifying characteristic is a characteristic that easily permits the identification of an instance of a concept with the correct concept.

Identification is  a distinct use case (possibly a set of distinct use cases) that definitional work must support.  It is not automatically supported by finding essential characteristics, as our example of Exit Row shows.  We shall return to the topic of identification in the future.  Frankly, identification seems to be poorly supported in the literature of definitions.  Yet it is something we are challenged with every day. 

If practical work with a concept involves identification of instances, the definition of the concept must support this work by clearly listing identifying characteristics and saying how they should be used in identification.


  1. Excellent examples.
    I am most familiar with the 'identification use case', and I find that when I review a model a major step is the definitional evaluation in which I ask myself whether I can look at a concrete instance of a 'thing' within the real world of the problem domain and determine whether that instance is unambiguously included or excluded from the defined set based solely on the definition provided.

  2. Malcolm, identification of a thing is also dependent on the application. For example, I myself, I have various identifications in various applications: for my mail system, it will be my e-mail address (possibly my alternate e-mail address or some passcode). However, if I want to pass through customs, I will need my passport or identity card with its specific ID. I cannot substitute one for the other - they are meaningless for the other application.

    Different contexts (applications) can have different identifying characteristics, or if you look at an identifying characteristic as a rule, different applications will be represented by different rule sets.

    Trying to capture all identifying characteristics in a definition will inevitably bring conflict between the parties (representing the applications) you are trying to reconcile. Separate capturing a definition from capturing all its rules, because commonalities and differences will always exists.